Electroluminescent device



6, 1966 R. s. LYNCH ETAL 3,267,318

ELECTROLUMINESCENT DEVICE Filed April 5, 1963 INVENTORS Rona/d 5.1 ml: Bradford .Sm/IIZ A'ITORNEY United States Patent 3,267,318 ELECTRGLUMINESCENT DEVICE Ronfld S. Lynch, Seneca Falls, N.Y., and Bradford K. Smith, Lititz, Pa, assignors to Sylvania Electric Hoducts Inc., a corporation of Delaware Filed Apr. 3, 1963, Ser. No. 270,284 3 Claims. (Cl. 313-408) This invention relates to an electroluminescent device and more particularly to a laminated electroluminescent structure wherein contiguous layers of diverse dielectric materials containing embedded phosphors are sandwiched between two separate electrodes.

Electroluminescent lamps are commonly fabricated by embedding electric field responsive phosphors in a translucent high dielectric material sandwiched between two electrically conductive surfaces or electrodes; one or both of which may be transparent depending upon the particular structure and intended utilization. To impart rigidity to the device one of the electrodes is usually disposed on a substrate or solid backing such as metal or ceramic. The application of a suitable electric potential to the respective electrodes creates a capacitive effect across the dielectric therebetween and activates the electric field responsive phosphors embedded therein to a state of luminescence.

Both plastics and ceramics have been separately and successfully utilized as the dielectric materials in electroluminescent devices. The selection of the one material over the other being determined by the specific results desired or the dictates of manufacturing economics.

Ceramic dielectrics are usually chosen where ruggedness and optimum moisture protection of the phosphors are desired. Electroluminescent lamps having a single layer of phosphor containing dielectric have been successfully fabricated but the possibilities of dielectric breakdown resulting from minute pinholes or voids in the single layer has led to the contiguous disposition of two or more dielectric layers, one upon the other, to materially strengthen the overall dielectric characteristics. Since each layer of ceramic dielectric is the resultant of applying and sintering a mixture of phosphor particles and ceramic powder, the build-up of each successive layer subjects the initially embedded phosphors to repeated heatings within a temperature range that is critical to phosphor stability. Thus these repeated heatings often have deleterious effects upon the lamp efficiencies by reducing phosphor responsive brightness and life.

Accordingly, it is an object of the invention to reduce the aforementioned disadvantages and to improve the fabrication of electroluminescent lamps.

A further object is to produce an electroluminescent device wherein the fabricating temperatures are conducive for utilizing the optimum long life characteristics of the incorporated phosphors.

An additional object is to fabricate a lamp that achieves optimum brightness of the phosphors embedded therein.

The foregoing objects are achieved in one aspect of the invention by the provision of a laminated electroluminescent structure wherein a ceramic dielectric layer, having electric field responsive phosphors embedded therein, is covered with a layer of non-vitreous low temperature cure plastic dielectric. The two contiguously associated dielectric layers are sandwiched between two electrically conductive layers or electrodes.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the accompanying figure which shows an enlarged cross sectional perspective of one embodiment of the device illustrating the invention.

- vitreous synthetic low cure temperature resin.

Patented August 16, 1966 Referring to the figure, there is shown a laminated e1ectroluminescent lamp 12. The substrate 14 is a solid backing material forming the foundation for the build-up of the laminated structure. This substrate material may be insulated metal, ceramic, glass or other insulative material depending upon whether opacity, translucency or transparency is to be utilized in the structure. In this particular instance transparency is desired, and therefore glass has been chosen.

A thin transparent electrically conductive coating or layer 16 is suitably disposed on one surface of the substrate 14 to form a first conductive layer or electrode.

A mixture of glass frit blended with suitable electric field responsive phosphors 20 in a suspending liquid is next applied, and upon heating becomes a vitrified ceramic dielectric layer 18 having the phosphor particles 20 embedded therein.

The second diele-ctric layer 22 of appropriate plastic resin is next disposed on the surface of dielectric layer 18. This dielectric layer 22 may or may not contain phosphor particles. As shown in the figure, it does not contain such particles in the embodiment presented.

The plastic, which has high dielectric properties and a coefficient of thermal expansion compatible with that of the contiguous ceramic dielectric layer 18, is a non- It also exhibits the characteristics of being resistant to organic solvents, moisture penetration and abrasion. The resin fills in any minute voids or pinholes existent in the ceramic layer 18 and further strengthens the dielectric characteristics thereof. It also functions as an additional sealant against the penetration of moisture, thus affording beneficial protection for the light output capabilities of the phosphors which are highly sensitive to moisture deterioration. Since this plastic is of the low cure temperature variety (10()200 C.), the phosphors embedded in the contiguous ceramic layer 18 are not deleteriously affected by critical repetitive heating.

The second electrically conductive layer 24 may be vapor deposited over plastic dielectric layer 22 and formed into a discrete pattern by conventional photoetching techniques. It can thus be seen that the plastic resin comprising dielectric layer'22 should be resistant to the chemicals encountered during photoetching. Since not all plastics individually exhibit the explicit characteristics desired for the described application, it is evident that the proper choice is necessarily restrictive to certain groups of resins to be hereinafter described.

To functionallyactivate this type of electroluminescent device, an external A. C. electromotive force source 26 is conductively connected across the first and second electrode layers 16 and 24 respectively. The capacitive effects initiated by this applied voltage across the dielectric layers activate the electric field responsive phosphors embedded therein to a state of luminescence.

In referring to the figure, there will be described in greater detail a specific embodiment of the device which is operable under typical conditions, as for example, 250 V. AC. at 400 c.p.s. as produced by electromotive force source 26.

The substrate 14 not only functions as the structural foundation but also serves as the Window or face plate of the device. As such it may be a conventional piece of transparent tempered soda lime glass having a thickness of approximately .125 inch and a light transmission characteristic for optimum viewing contrast.

After a cleaning operation the substrate 14 has a thin transparent electrically conductive layer, constituting the first electrode 16, suitably disposed on one surface thereof in accordance with one of several conventional procedures. One such accepted method involves heating the substrate to approximately 600 C. and spraying the heated substrate surface with a stannic chloride solution, whereby there is deposited thereon a thin film of transparent electrically conductive tin oxide of a thickness equal to approximately 500 ohms per square, that is a resistance of 500 ohms taken between the entire opposite sides of any square on the tin oxide film.

Materials for forming the ceramic dielectric layer 18, having particles of field responsive phosphors 20 dispersed therein, are next applied over the first conductive layer 16. The basic ceramic material is a finely divided glass powder or frit desirably free of reducible metallic compounds, particularly those containing lead, which would produce unwanted opacity and increased conductivity. Also the vitrification or fusion temperature of the frit must be below that which would be harmful to the field responsive phosphors associated therewith. The glass frit is thoroughly blended with a suitable electric field responsive electroluminescent phosphor, such as copper activated zinc sulfide, in a suspended liquid as, for example, isopropyl alcohol and hexylene glycol. This liquid mixture may then be disposed on the first conductive layer 16 by a suitable technique, such as spray application whereby the material deposition is approximately 100 mg./sq. in. The substrate 14 containing the first conductive layer 16 and the materials for dielectric layer 18 are furnace heated to a temperature of approximately 675 C. for a time interval sufficient to fuse or vitrify the frit. Thus there is formed the first dielectric ceramic layer 18 having the field responsive phosphor particles 20 embedded therein.

Upon layer 18 there is next disposed a second dielectric layer 22 of plastic resin having a thermal coetficient of expansion compatible with that of the ceramic. In this particular structure the plastic dielectric layer 22 does not contain embedded :phophors, although in some embodiments phosphors may be included. The dielectric thickness of the electroluminescent structure is desirably increased by having this plastic layer directly overlay the ceramic dielectric layer 18. The actual thickness of the plastic is determined by the overall dielectric strength required for the successful operation of the device. In this embodiment a thickness of 1 to 2 mils is sufficient cover to fulfill the desired dielectric requirements, fill in any existent pinholes in the ceramic layer and afford further protection against moisture penetration. The foregoing requirements, in addition to the needs for a low cure temperature resin that is resistant to organic solvents and abrasion, are adequately fulfilled by certain of the synthetic thermo-setting resins from the polyester, epoxy and silicone groupings.

In applications where lower temperatures may be encountered (under 100 C.) some of the thermoplastics such as for example the acrylics, may be satisfactorily utilized.

Optical requirements of the plastic will vary in accordance with the usage. In some cases transparency will be desired and in others reflectivity. In the device portrayed in the figure, light radiation emanating from the activated phosphor particles 20 in dielectric layer 18 passes through the first conductive layer 16 and thence through the transparent substrate 14. This plastic dielectric layer 22 further functions as a back-up layer for dielectric layer 18 and may beneficially exhibit reflectivity.

The plastic resins have been successfully applied by a spray technique after which a short heating step within the 100 C.200 C. temperature range adequately cures the material to the desired hardness for disposing the second electrode layer 24 thereon. An epoxy material suitable for this usage is A-423 Epoxy Gloss White with thinner catalyst as manufactured by the Andrew Brown Company, Laurel, Maryland. A suitable silicone plastic material is P and S Clear Dielectric Coating A-8983 with catalyst as prepared by Pierce and Stevens Chemical Corporation, Buffalo, New York. An acrylic satisfactory for lower temperature applications is Glossy White Krylon #1501 as formulated by Krylon, Inc., Norristown, Pennsylvania.

After the plastic layer 22 has hardened, the second electrode or second electrically conductive layer 24 is applied by conventional means such as the vaporization of aluminum in a vacuum. It is often desired to have this electrode in the form of a discrete pattern which can be adequately formed by a conventional photoetching technique. As stated above, the plastic of layer 22 is selected to withstand the chemical deterioration normally involved in photoetching.

There has thus been described a laminated electrolurninescent device that has the dielectric portion advantageously formed of both ceramic and plastic. Ruggedness and moisture resistance is achieved by the ceramic layer which is further enhanced by the plastic overlay. Optimum brightness and life characteristics of the phos phors embedded in the ceramic are achived by limiting the heating to which the phosphors are exposed.

While there has been shown and described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. A laminated electroluminescent device comprising:

a first electrically conductive layer forming a first electrode;

a first dielectric layer of ceramic having electric field responsive phosphors embedded therein covering the surface of said first electrode, said first dielectric having pinholes in the surface thereof;

sealing means in the form of a second dielectric layer of a substantially gloss white synthetic plastic resin overlying said ceramic layer and filling in said pin holes in the surface of said first dielectric layer, said plastic resin having a curing temperature of less than 200 C. and having light reflective properties to enhance the light radiation emanating from said phosphors; and

a second electrically conductive layer forming a second electrode overlaying said second dielectric layer.

2. A laminated electroluminescent device according to claim 1 wherein said plastic resin is a substantially gloss white epoxy material.

3. A laminated electroluminescent device according to claim 1 wherein said plastic resin is a substantially gloss white acrylic material.

References Cited by the Examiner UNITED STATES PATENTS 2,824,992 2/ 1958 Bouchard et al 313-108 2,964,666 12/1960 Klasens et al 313l08 3,004,931 10/1961 Brueschweiler et al 117124 3,037,138 5/1962 Motson 313-108 3,037,955 6/1962 Carman 117124 3,046,433 7/1962 Browne 313-108 3,088,933 5/1963 Ot'tmann et a1. 1l7124 3,096,458 7/ 1963 Demmy 313-108 3,129,108 4/1964 Katona 117-33.5 3,186,965 6/1965 Plueddemann 117123 3,205,393 9/1965 Mash 313108 JAMES W. LAWRENCE, Primary Examiner.

GEORGE N. WESTBY, DAVID I GALVIN,

Examiners. R. JUDD, Assistant Examiner. 

1. A LAMINATED ELECTROLUMINESCENT DEVICE COMPRISING: A FIRST ELECTRICALLY CONDUCTIVE LAYER FORMING A FIRST ELECTRODE; A FIRST DIELECTRIC LAYER OF CERAMIC HAVING ELECTRIC FIELD RESPONSIVE PHOSPHORS EMBEDDED THEREIN COVERING THE SURFACE OF SAID FIRST ELECTRODE, SAID FIRST DIELECTRIC HAVING PINHOLES IN THE SURFACE THEREOF; SEALING MEANS IN THE FORM OF A SECOND DIELECTRIC LAYER OF A SUBSTANTIALLY GLOSS WHITE SYNTHETIC PLASTIC RESIN OVERLYING SAID CERAMIC LAYER AND FILLING IN SAID PINHOLES IN THE SURFACE OF SAID FIRST DIELECTRIC LAYER, SAID PLASTIC RESIN HAVING A CURING TEMPERATURE OF LESS THAN 200*C. AND HAVING LIGHT REFLECTIVE PROPERTIES TO ENHANCE THE LIGHT RADIATION EMANATING FROM SAID OPHOSPHORS; AND A SECOND ELECTRICALLY CONDUCTIVE LAYER FORMING A SECOND ELECTRODE OVERLAYING SAID SECOND DIELECTRIC LAYER. 